1. Field of Invention:
This invention relates generally to masonry anchors, and more particularly to a self-tapping, screw-type masonry anchor which can readily be turned into a hole bored in a masonry structure to secure a fixture or other object thereto, the installed anchor being highly resistant to forces seeking to pull the anchor out of the hole.
2. Status of Prior Art:
It is frequently necessary to secure fixtures, brackets, channel pieces and other more or less heavy objects to the surface of a masonry structure formed of concrete, brickwork or other masonry material. These objects are fastened to the masonry structure by masonry anchors.
Concrete is made by mixing cement and an aggregate of inert particles of varying size, such as a combination of sand or broken stone screenings with gravel. Compressive strength is generally accepted as the principal index to the structural quality of concrete. Mixtures for concrete masonry structures, such as walls and partitions, ordinarily employ aggregates having a maximum size of one-half inch. Masonry brick is usually formed from clay, shale or pumice hardened by heat. Bricks for this purpose are available in different degrees of harness, depending on the material used in making the brick.
But regardless of the material used in forming a masonry structure, when a hole is drilled therein to receive a metal masonry anchor whose threaded shank taps the wall of the hole, because the material is friable, the tapping action causes the wall to crumble or pulverize.
The patent to Ernst, U.S. Pat. No. 3,937,119, discloses a self-tapping, screw-type metal anchor having a sharp-crested helical male thread surrounding the shank of the anchor, the convolutions of the thread running the length of the shank. This male thread, when the anchor is screwed into a hole drilled in masonry, functions to tape the wall of the hole to create an internal female thread. Since the male thread on the shank mates with the female wall thread, the anchor is then resistant to pull-out forces which seek to back the anchor out of the hole.
Also included in the Ernst anchor is a second thread whose convolutions surround the shank in the successive spaces between the convolutions of the cutting thread. The crests of the second thread have a diameter smaller than that of the cutting thread crests and about equal to the diameter of the masonry hole. This second thread functions as a guide thread to center the anchor in the masonry hole, so that the anchor is not permitted to tilt as it is turned into the hole. By preventing tilting of the anchor in the masonry hole, one enhances its pull-out resistance; for if the anchor were tilted, the crests of the cutting thread would then not be properly embedded in the hole.
In a preferred form of the Ernst masonry anchor, the crests of the cutting thread are not of uniform diameter throughout the length of the shank, but decrease progressively from the trailing end of the shank toward the tip or leading end. According to Ernst, an anchor having a tapered cutting thread can be turned into a masonry hole with a minimal amount of torque by means of a conventional screwdriver.
Another feature of the Ernst masonry anchor resides in a series of notches formed on the crests of the cutting thread along the full length of the shank. These notches, in conjunction with the tapering of the crest diameters of the cutting thread, make it possible to insert the anchor in the masonry hole without unduly disturbing the wall of the hole. Masonry material tends to crumble when worked. But with the gradual chipping carried out by the notches during continued rotation of the anchor, this serves to ensure the firm embedment in the wall of at least the relatively large diameter crests at the trailing end of the cutting thread. According to Ernst, as aggregate particles of the masonry hole wall are gradually removed from the wall by the action of the notches, the material so removed is trapped in the spaces between the convolutions of the cutting thread. The retention of these particles in the spaces enhances the pull-out strength of the anchor.
We have found, however, that an anchor of the Ernest type actually offers low resistance to back out resulting from vibratory and other external forces transmitted to the installed anchor through the masonry structure. These forces may originate from operating machinery mounted on the masonry structure, or heavy moving trucks or other vehicles traveling near the site of the masonry structure. Also, high winds are a factor when the masonry structure is exposed thereto. These forces seek to back the installed anchor out of the masonry hole.
The reason Ernst's masonry anchor, which has a tapered male cutting thread, has poor back-out resistance, is that when turned into a masonry hole to tape the wall of this hole, it then creates on this wall a tapered internal female thread that mates with the tapered male thread. When a male tapered element is fully socketed within a complementary female tapered element, these elements are then contiguous. But if the male element is axially withdrawn even to a slight degree, then the tapered male element is separated from the complementary female element.
An Ernst anchor having a tapered male cutting thread, in its installed state, is intermeshed with the tapered internal female thread in the wall of the masonry hole. If, therefore, the Ernst anchor, which serves to hold an object against the masonry surface, is subjected to vibratory forces of sufficient strength to back the anchor out of the hole even to a slight degree, when this happens the male thread disengages from the female thread. The anchor is then loose and is free to rotate out of the hole, and it ceases, therefore, to hold the object against the masonry.
Another drawback of the Ernst screw-type masonry anchor whose cutting thread has a tapered formation is that it requires a high degree of torque to drive the anchor into the masonry hole. The reason for this is that the diameter of the cutting crests increases progressively from the leading to the trailing end of the shank. Hence, as the anchor is rotated to tap its way into the masonry hole, the wall of the hole is engaged by crests of increasing diameter, this resulting in a progressive rise in drag and greater resistance to rotation.
The high torque required to turn the Ernst masonry anchor into the masonry hole may limit the depth of entry, for a point is reached where even the high torque is insufficient to overcome the resistance to turning, and further advance of the anchor into the hole is arrested. And when the anchor is so arrested, the continued application of a high torque may cause the anchor to fracture or snap in half.
My above-identified copending application discloses a self-tapping, screw-type masonry anchor which, when installed in a hole drilled in a masonry structure to secure an object thereto, is then highly resistant to pull-out forces. This anchor is fabricated of a metal whose harness is such that when it rubs against a masonry surface it is abraded and worn thereby. The anchor includes a head engageable by a torque-producing tool to rotate the anchor, and a shank extending therefrom provided with a root section surrounded by a helical male cutting thread whose series of convolutions extend from the leading to the trailing end of the root section have sharp crests. The crests have diameters greater than that of the hole, the crest diameters decreasing progressively from the leading to the trailing end to create a reverse taper.
When the reverse taper anchor is rotated to cause it to enter the hole and tap its wall, the sharp crest at the leading end of the cutting thread experiences a high degree of wear and blunting, the degree of crest wear diminishing progressively toward the trailing end to effectively erase the taper. Thus, in the installed anchor, the male thread is of substantially uniform diameter and intermeshes with a mating female internal thread in the wall of the hole, this relationship resulting in high resistance to loosening of the anchor.
The effectiveness of a reverse-taper anchor of the type disclosed in my copending application depends on the nature of the masonry in which the anchor is installed. Its nature must be such in relationship to the hardness of the anchor metal as to wear down and blunt the sharp crests of the reverse-taper male thread, so that when the anchor is installed, the taper is then erased. However, in some cases, the abrasive nature of the concrete, or whatever other form of masonry is involved, may be such as to give rise to little wear or to produce excessive wear of the crests. Hence, while in most masonry installations the wearable reverse-taper anchor is highly effective; in some cases, it is less so.
Of prior art background interest are the following references: The U.S. Pat. Nos. 3,351,115 (Boehlow); 4,842,467 (Armstrong); 4,439,077 (Godstead); 5,044,853 (Dicke) and German patent 2,815,247 to Fischer (1979).